1. Field of the Invention
The invention generally concerns electronic dimming ballasts which are responsive to a phase angle controlled AC input to control the illumination level of gas discharge lamps. More particularly, the invention concerns measures which improve the interaction between an EMI filter of the ballast and the phase control dimmer.
2. Description of the Prior Art
Electronic dimming ballasts are commercially available in which dimming of gas discharge lamps, typically fluorescent lamps, is responsive to phase angle control of the AC power line input. Phase angle control involves the clipping of a portion of each half cycle of the AC sinusoidal power line voltage. A common type of phase angle controller, known generally as a forward phase dimmer, clips or blocks a portion of each half cycle immediately after the zero crossing. An example of a forward phase dimmer is the well known triac dimmer. Another type is the reverse phase dimmer, commonly known as an electronic dimmer, which passes the portion of the half-cycle immediately after the zero crossing and blocks the portion of the half cycle before the zero-crossing. In both types, the portion or angle of the half cycle which is blocked is adjustable.
Various dimming ballasts are known which employ a dim input terminal which is separate from the input terminals for the mains supply, and are generally known as three-wire dimming ballasts. Examples of such ballasts are known from JP-116698, DGM 9014982, and U.S. Pat. No. 4,797,599. Incandescent lamps are typically dimmed with only two wires from a phase control dimmer, i.e. the common lead and the hot dimmed lead which carries the phase control information. The above three-wire ballasts are inconvenient in that, when installing a fluorescent ballast and lamp in place of incandescent lamps, an additional wire must be run from the phase control dimmer (typically wall mounted) to the ballast (typically ceiling mounted). This results in considerable labor costs and is an impediment to market acceptance.
Two wire dimming ballasts are more attractive from an installation standpoint. Examples are known from U.S. Pat. No. 4,392,086 (Ide et al) and U.S. Pat. No. 5,192,896 (Qin). As with non-dimming ballasts, it is desirable to have an EMI filter to prevent high frequency components generated in the ballast from entering the power line.
U.S. Pat. No. 4,492,897 (Sairanen) discloses interaction problems between the EMI filter and the phase control dimmer in a two-wire dimming ballast. Sairanen's ballast has an EMI LC filter which includes a choke and a first filter capacitor connected at the input of a full-bridge rectifier, and a second, large-scale electrolytic filter capacitor connected across the output of the rectifier. The electrolytic capacitor supplies power to the lamp circuit in addition to the filter function. Sairanen discloses that if the lamp is removed or if the lamp circuit otherwise draws little power, the voltage across the two filter capacitors can rise to a dangerously high level. Sairanen discloses that this over-voltage on the filter capacitors, when the electrolytic filter capacitor is not fully loaded, is caused by the resonant frequency of the choke and the first filter capacitor being higher than the mains frequency. This leads to oscillation of the EMI filter with the output voltage of the phase angle controller. To overcome this problem, Sairanen includes a switch which switches out the first filter capacitor when the large-scale electrolytic capacitor is not sufficiently loaded, such as when the lamp current is at low levels. Sairanen alternatively discloses taking the first filter capacitor out of the circuit by an artificial load, such as a PTC resistor.
The disadvantage of the Sairanen solution is that taking the first filter capacitor out of the circuit disables the EMI filter, so high frequency interference will be introduced into the power lines during ballast operation.
Accordingly, it is the object of the invention to provide a two-wire dimmable ballast having an EMI filter with improved operation with a phase control dimmer, and in particular, with a triac dimmer.